The present invention relates to digital image processing including scaling and color conversion of RGB signals to CMYK format, such as for printing high quality images from digital cameras and the like.
In the processing of graphic images, it is often desired or required to alter the magnification. Traditionally, optical means such as interchangeable lenses, zoom lenses, and mechanical positioners have been used on copy cameras and photo-copiers of the prior art. More recently, photo-copiers have incorporated linear sensor arrays that are sequentially and repetitively read in a main scan direction as the arrays are moved across the page in a sub-scan direction. Numerous systems have been developed for selectively changing the magnification electronically, such as by using a computer programmed with appropriate software. Such systems are prohibitively slow in many applications. U.S. Pat. No. 4,920,571 to Abe et al. discloses an image processor that incorporates an interpolator that uses look-up tables for adding image data in enlarging operations and selectively reducing the data in reducing operations. In one example, the magnification can range by 1/64 ths from 32/64 to 128/64 (0.5-2.0). The processor of Abe et al. has the disadvantage of requiring a separate look-up table for each magnification. Thus it is ineffective in that it is capable of only a limited choice of magnifications; and/or it is overly complex, requiring a multiplicity of look-up tables.
U.S. Pat. No. 5,535,007 to Kim discloses a pipeline processor for enlarging or reducing an original image to obtain a copy image scaled in 1 percent increments, with sharpening and color conversion. The device includes means for inserting or removing pixel data in a main-scanning direction and inserting or omitting the scanning lines using separate look-up tables that are populated according to designated image input parameters and control factors that correspond to a desired magnification, which can typically range from 50% to 400%. Also disclosed are conversion from RGB to CMYK color formats by a CMY look-up table that feeds an under color removal (UCR) comparator and subtractor, and a color corrector that selectively substitutes converted color reference values for C, M, and Y, depending on a comparison of input data with respective register values. The processor of Kim, while enabling scaling in one-percent increments, has the disadvantage of having to populate the look-up tables for each different magnification, thereby adding complexity to the processor and slowing the process.
U.S. Pat. No. 5,867,634 to Hirota et al. discloses an image processor in which an image of a document is read by an image sensor, and a reduction ratio of the image is set. Matrix interpolation (for reducing magnifications) is then performed on the image data between a pixel under interest and pixels adjacent thereto as functions of the reduction ratio. Next, reduction/magnification is carried out by sequentially reading stored interpolation data at a read clock rate that is proportional to a write clock rate at which the interpolation data is stored. The data is read into eight smoothing filters that are configured for separate integer ranges of positive magnification, the data from one of the filters being selected as output depending on the actual magnification. Color conversion is done in stages, first in logarithmic conversion by table look-ups from reflective to density RGB values, followed by UCR/BP processing and color correction by matrix masking. The image processor of Hirota, unfortunately, is excessively complex, requiring a number of variable multiplication operations (including squaring) to be performed on multiple channels of data for each pixel to be processed. Also, the scaling subsystem effectively requires images to be greatly enlarged, then reduced.
Thus there is a need for an image processing system that overcomes the disadvantages of the prior art.